Identifying angle of departure of multi-antenna transmitters

ABSTRACT

A method for signal processing includes receiving at a given location at least first and second signals transmitted respectively from at least first and second antennas (34) of a wireless transmitter (24). The at least first and second signals encode identical data using a multi-carrier encoding scheme with a predefined cyclic delay between the transmitted signals. The received first and second signals are processed, using the cyclic delay, in order to derive a measure of a phase delay between the first and second signals. Based on the measure of the phase delay, an angle of departure (θ) of the first and second signals from the wireless transmitter to the given location is estimated.

FIELD OF THE INVENTION

The present invention relates generally to wireless communicationsystems, and particularly to methods for localization based on wirelessnetwork signals.

BACKGROUND

Various techniques are known in the art for finding the location of amobile wireless transceiver, such as a cellular telephone. For example,nearly all cellular telephones now have a Global Positioning System(GPS) receiver, which derives location coordinates from signals receivedfrom geostationary satellites. Because of its dependence on weaksatellite signals, however, GPS works poorly, if at all, indoors and incrowded urban environments. Cellular networks are also capable oftriangulating telephone location based on signals received ortransmitted between the cellular telephone and multiple cellularantennas, but this technique is inaccurate and unreliable.

A number of methods have been proposed for indoor localization based onan existing wireless local area network (WLAN) infrastructure. One suchapproach is described, for example, by Kotaru et al., in “SpotFi:Decimeter Level Localization using WiFi,” published in SIGCOMM '15(London, UK, Aug. 17-21, 2015). According to the authors, SpotFicomputes the angle of arrival (AoA) of multipath components receivedfrom access points, and uses filtering and estimation techniques toidentify the AoA of a direct path between the localization target andthe access point.

As another example, U.S. Patent Application Publication 2009/0243932describes a method for determining the location of a mobile device. Themethod comprises transmitting a signal between a plurality of knownlocations and receiving the signal at a device of unknown location, suchas a mobile device. The signal may include multiple tones havingdifferent frequencies and resulting in sets of residual phasedifferences. The location of the mobile device may be determined usingthe known locations and the frequency and phase differences between thetransmitted tones. In one embodiment, OFDM signals may be used betweenan access point and mobile device to determine the location of themobile device.

As a further example, U.S. Patent Application Publication 2016/0033614describes a method of direction finding (DF) positioning involving mainlobe and grating lobe identification in a wireless communication networkis proposed. A receiver performs the DF algorithm on radio signalsassociated with multiple antennas over a first channel frequency andestimates a first set of DF solutions. The receiver performs the DFalgorithm on radio signals associated with multiple antennas over asecond channel frequency and estimates a second set of DF solutions. Thereceiver then identifies the correct DF solution (e.g., the main lobedirection) by comparing the first set of DF solutions and the second setof DF solutions. Most current WLANs operate in accordance with the setof 802.11 standards promulgated by the IEEE. Within this family, theIEEE 802.11n-2009 standard (commonly referred to simply as “802.11n”)defines the use of multiple antennas to increase data rates by means of“multiple input and multiple output” (MIMO) transmission and reception.MIMO enables the transmitter and receiver to coherently resolve moreinformation than would be possible using a single antenna, by means ofspatial division multiplexing (SDM), which spatially multiplexesmultiple independent data streams within one spectral channel ofbandwidth. The newer 802.11ac standard similarly supports MIMOtransmission, with a larger number of spatial streams and highertransmission rates than 802.11n.

SUMMARY

Some embodiments of the present invention that are described hereinbelowprovide improved methods for extracting directional information fromwireless access point signals, as well as devices and systems thatderive and make use of such information.

There is therefore provided, in accordance with an embodiment of theinvention, a method for signal processing, which includes receiving at agiven location at least first and second signals transmittedrespectively from at least first and second antennas of a wirelesstransmitter. The at least first and second signals encode identical datausing a multi-carrier encoding scheme with a predefined cyclic delaybetween the transmitted signals. The received first and second signalsare processed, using the cyclic delay, in order to derive a measure of aphase delay between the first and second signals. Based on the measureof the phase delay, an angle of departure of the first and secondsignals from the wireless transmitter to the given location isestimated.

In some embodiments, receiving the at least first and second signalsincludes receiving at least the first and second signals via a singlereceiving antenna, such as an omnidirectional antenna installed in amobile telephone.

Additionally or alternatively, the multi-carrier encoding schemeincludes an orthogonal frequency-domain multiplexing (OFDM) scheme. Insome embodiment, the transmitter is a wireless access point operating inaccordance with an 802.11 standard.

In the disclosed embodiments, processing the received first and secondsignals includes selecting at least two time-frequency bins in thereceived signals, and computing the measure of the phase delayresponsively to respective cyclic shifts of the selected bins. In someembodiments, selecting the at least two time-frequency bins includessampling first and second bins at a selected frequency within themulti-carrier encoding scheme in different, respective first and secondsymbols within a frame transmitted by the transmitter. In otherembodiments, selecting the at least two time-frequency bins includessampling first and second bins at different, respective first and secondfrequencies within the multi-carrier encoding scheme in a selectedsymbol within a frame transmitted by the transmitter.

In a disclosed embodiment, selecting the at least two time-frequencybins includes selecting first and second bins having antipodal phasesbased on standard cyclic shifts. Additionally or alternatively,computing the measure of the phase delay includes applying a lineartransformation to the signals extracted from the selected time-frequencybins.

In other embodiments, processing the received first and second signalsincludes computing a temporal correlation function over the first andsecond signals, and finding the measure of the phase delay responsivelyto a time difference between peaks in the temporal correlation function.In a disclosed embodiment, the correlation is computed over one or moresymbols that repeat in the signals with a predefined period inaccordance with the wireless communication standard, whereby the peaksin the temporal correlation function have a periodicity corresponding tothe predefined period, and finding the measure includes identifying,responsively to the periodicity, one or more pairs of the peaks suchthat the time difference between the peaks in each of the pairscorresponds to the predefined cyclic delay.

The temporal correlation function may be selected from a group offunctions consisting of an autocorrelation function and across-correlation with a predefined reference signal. Additionally oralternatively, the method includes finding a number and/or order of theantennas transmitting the signals from the wireless transmitter based onthe number of peaks and their relative locations in the temporalcorrelation function. The correlation may be computed over at least apart of the preamble of a given frame for processing in the receivedsignals, for example over one or more synchronization symbols in thepreamble that are defined by a wireless communication standard.

In some embodiments, the first and second signals are transmitted inaccordance with a wireless communication standard that specifies a framestructure including a predefined preamble, and processing the receivedfirst and second signals includes selecting at least a part of thepreamble of a given frame for processing.

Additionally or alternatively, receiving and processing the at leastfirst and second signals include receiving and processing at least thefirst and second signals in a mobile station without establishing anassociation between the mobile station and the access point. In oneembodiment, receiving and processing at least the first and secondsignals includes receiving a beacon transmitted from the at least firstand second antennas in accordance with a predefined wirelesscommunication standard.

In some embodiments, the method includes receiving location informationwith respect to the wireless access point, and computing coordinates ofthe given location based on the received location information and theestimated angle of departure. Typically, computing the coordinatesincludes finding the coordinates based on the received locationinformation and the estimated angle of departure with respect to two ormore different wireless access points.

Additionally or alternatively, the method includes extracting anidentifier of the wireless access point from at least one of thereceived first and second signals. In one embodiment, the methodincludes reporting the identifier and the estimated angle of departureto a server, for incorporation into a map containing respectivelocations of multiple access points.

There is also provided, in accordance with an embodiment of theinvention, a method for mapping, which includes receiving reports, froma set of wireless communication devices at respective locations, ofrespective estimated angles of departure of signals received by thewireless communication devices from wireless access points. A map of thewireless access points is constructed based on the estimated angles ofdeparture.

In a disclosed embodiment, the wireless communication devices includemobile stations having a single antenna, while the wireless accesspoints each have multiple antennas, and the angles of departure areestimated based on a predefined cyclic delay between the signalstransmitted by the multiple antennas. Additionally or alternatively,receiving the reports includes receiving from the wireless communicationdevices respective identifiers of the wireless access points andlocation coordinates of the wireless communication devices where thesignals were received.

In some embodiments, the method includes providing location informationfrom the map to one or more of the wireless communication devices. Inone embodiment, providing the location information includes identifyingposition coordinates of a wireless communication device based on anestimated angle of departure of signals received by the wirelesscommunication devices from a given access point and a location of thegiven access point in the map.

There is additionally provided, in accordance with an embodiment of theinvention, a wireless device, including a receive antenna, configured toreceive at a given location at least first and second signalstransmitted respectively from at least first and second antennas of awireless transmitter. The at least first and second signals encodeidentical data using a multi-carrier encoding scheme with a predefinedcyclic delay between the transmitted signals. Processing circuitry isconfigured to process the received first and second signals, using thecyclic delay, in order to derive a measure of a phase delay between thefirst and second signals, and to estimate, based on the measure of thephase delay, an angle of departure of the first and second signals fromthe wireless transmitter to the given location. There is furtherprovided, in accordance with an embodiment of the invention, apparatusfor mapping, including a memory and a processor, which is configured toreceive reports, from a set of wireless communication devices atrespective locations, of respective estimated angles of departure ofsignals received by the wireless communication devices from wirelessaccess points, and to construct, in the memory, a map of the wirelessaccess points based on the estimated angles of departure.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic, pictorial illustration of a system for wirelesscommunications and position finding, in accordance with an embodiment ofthe invention;

FIG. 2 is a diagram that schematically illustrates a coordinate frameused in deriving an angle of departure of wireless signals from anaccess point to a receiver, in accordance with an embodiment of theinvention;

FIG. 3 is a block diagram that schematically illustrates components of amobile receiver that are used in deriving coordinate information withrespect to wireless access points, in accordance with an embodiment ofthe invention;

FIG. 4 is a block diagram that schematically illustrates components of amobile receiver that are used in deriving coordinate information withrespect to wireless access points, in accordance with another embodimentof the invention; and

FIG. 5 is a schematic, pictorial illustration of components of thesystem of FIG. 1, illustrating a method for finding the position of amobile communication device, in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Implementation of MIMO SDM schemes, such as those defined by the 802.11nstandard, requires a discrete antenna at both the transmitter and thereceiver for each spatial stream. Alternatively, the phases of thestreams can be adjusted to enable directional beamforming between theaccess point and mobile station, thus maximizing the signal at thereceiver. Many mobile devices, however, such as WiFi-enabledsmartphones, have only a single, omnidirectional antenna, and thuscannot themselves support MIMO or directional transmission.

In addition to actively forming beams toward the receiver, the 802.11nstandard also defines a scheme for preventing unintentional beamforming,which can occur if the data transmitted in the various spatial streamsinadvertently form correlated patterns, for example if the number ofstreams is smaller than the number of antennas. Unlike intentionalbeamforming, the pattern of lobes and nulls created by unintentionalbeamforming may not be oriented in such a way as to maximize the signalat the receiver, and thus can be detrimental to reception. To avoidunintentional beamforming, the 802.11n standard applies a cyclic delaydiversity (CDD) to offset the IFFT coded data stream from each antennaby a different constant, non-coherent delay. In the multi-carriermodulation system mandated by the 802.11n standard—known as orthogonalfrequency domain multiplexing (OFDM)—CDD is applied by adding apredefined cyclic shift in time which affects each carrier on eachantenna and is referred to equivalently as cyclic shift diversity (CSD).

Although CDD was introduced as a means for avoiding undesireddirectionality of multi-antenna wireless signals, embodiments of thepresent invention that are described herein exploit the CDD in receivedsignals for just the opposite purpose: to estimate the angularorientation of the access point transmitting the signals. This angularorientation is defined in terms of the angle of departure, i.e., theangle between the direction of the transmitted beam and the axis of thetransmitting access point (as defined by a line drawn through thelocations of the transmitting antennas). This sort of measurement ofangle of departure from the transmitter is in contrast to techniquesthat are known in the art for determining the angle of arrival ofsignals at the receiver. The angle of departure can be found, using thetechniques described herein, even when the signals from the transmittingantennas are received via a single receiving antenna, such as the sortof omnidirectional antenna that is commonly installed in mobiletelephones.

The embodiments of the present invention that are described hereinbelowspecifically provide methods for finding the angle of departure ofmultiple signals that are transmitted respectively from multipleantennas of a wireless access point and are received at a givenlocation. The transmitted signals encode, at least in part, identicaldata, with a predefined, per antenna, cyclic delay between thetransmitted signals. The data are typically defined and transmitted inaccordance with a known wireless communication standard, so that atleast a part of the data, as well as the cyclic delay, are predictable.For example, the 802.11 standards specify a frame structure including apredefined preamble, which can be selected for processing by thereceiver in the present context.

The receiver processes the received signals, using the known cyclicdelay, in order to derive a measure of the actual phase shift betweenthe signals. This phase shift, in turn, is indicative of the differencein path lengths that the signals traverse in reaching the receiver. Thespacing between the transmitting antennas is also known: typically λ/2(half the wavelength of the transmitted radio signals). Thus, based onthe measure of the phase shift, along with the known distance betweenthe antennas and the known amount of cyclic delay, it is possible toestimate the angle of departure of the signals from the wireless accesspoint to the location of the receiver.

In some embodiments, the signals are multi-carrier signals, such as thesort of OFDM signals that are used in the 802.11n standard, and thecyclic delay is thus implemented by applying different, respectivecyclic shifts to different antennas. In this case, the receiver is ableto estimate the phase shift between the antennas by properly selectingtwo time-frequency bins in the received signals, and computing themeasure of the phase shift using the respective cyclic shifts of bins astransmitted by the multiple antennas. By judicious selection of thebins, it is possible to compute the angle of departure by applying, forexample, a simple linear transformation to the extracted frequency bins.For this purpose, it is advantageous that the bins be chosen, based onthe cyclic delays defined by the applicable standard, so as to haveantipodal cyclic shifts.

The term “time-frequency bin,” as used in the context of the presentdescription and in the claims, means a sample of the received signal ata given, predefined frequency taken at a given, predefined time from thestart of a frame transmitted by the transmitter. Two time-frequency binscan be at the same frequency or different frequencies, and can occur atthe same or different times. In 802.11 OFDM transmissions, for example,a bin is defined in terms of frequency as one of N predefined complexnumbers used in data encoding prior to conversion to the time domain byInverse Fast Fourier Transform (IFFT), such as N=64 for 20MHz 802.11OFDM. The input to the

IFFT encoder in OFDM systems (which transforms frequency-domain totime-domain signals) is a fixed-size collection of complex numbers, eachcorresponding to a frequency bins.

Thus, in some embodiments (referred to as “time domain” embodiments),the two bins are defined by sampling a selected frequency within themulti-carrier encoding scheme at different, respective OFDM symbols,which are coded by different respective cyclic (time) shifts within aframe transmitted by the transmitter. In other embodiments (referred toas “frequency domain” embodiments), the two bins are defined by samplingtwo different frequencies within the multi-carrier encoding scheme atthe same selected OFDM symbol within a transmitted frame.

In other embodiments, the receiver estimates the phase shift between theantennas by computing a temporal correlation function, such as anautocorrelation or a cross-correlation with a reference signal, andfinding peaks that are separated by an appropriate time difference(which is determined by the cyclic delay) in the correlation. Thecorrelation function can advantageously be computed over the framepreamble, particularly over synchronization symbols in the preamblehaving good correlation qualities, as defined by the applicablestandards. The numbers and expected locations of the peaks in thecorrelation can also be used in finding the number of antennas used bythe wireless transmitter in transmitting the signals.

Another advantage of the present techniques is that they are capable ofreceiving and processing the access point signals in a mobile station,in order to measure the angle of departure, without establishing anassociation between the mobile station and the access point. In otherwords, the mobile station can simply capture signals silently, withouttransmitting signals back to the access point, and can analyze thesignals using the known, standard preamble structure and cyclic delay.The mobile station can apply this analysis, for example, to beacons thatare transmitted by an access point, in accordance with certain 802.11standards, even when there are no mobile stations communicating with theaccess point. Alternatively, the mobile station can receive and analyzesignals that are directed to other mobile stations.

Although the angle of departure itself does not uniquely identify thelocation of the transmitting access point, multiple measurements ofangle of departure, from different, known receiver locations, can beused to find the access point location by triangulation. Thus, someembodiments of the present invention provide a method for mapping accesspoint locations by combining multiple measurements of angle of departuremade from different receiver locations. The signals transmitted by theaccess points also identify the access points (for example, byannouncing the Basic Service Set Identifier—BSSID), so that the identityof each access point can be associated with its location.

By the same token, when the locations of access points are known, it ispossible to find the location of a receiver by measuring the angles ofdeparture from two or more of these known access points to the receiver,using the techniques described above. Thus, once the locations of accesspoints in a certain area have been mapped, receivers, such as mobiletelephones, can find their own locations accurately within the areabased on the signals that they receive from the access points (evenwithout associating or otherwise communicating back with the accesspoints, as explained above). This sort of map of access points can thusbe used for accurate and convenient geo-location without dependence onGPS, for example in indoor and urban locations.

Although the embodiments described hereinbelow relate, for the sake ofconcreteness and clarity, specifically to 802.11 wireless access points,the principles of the present invention may similarly be applied,mutatis mutandis, to other sorts of multi-antenna transmitters thattransmit signals using multi-carrier encoding schemes. For example, inan alternative embodiment of the present invention, the receiver can beconfigured to measure angles of departure of multi-antenna cellular basestations that transmit OFDM signals in accordance with the applicablestandards. All such alternative implementations of the presentprinciples are considered to be within the scope of the presentinvention.

System Description

FIG. 1 is schematic, pictorial illustration of a system 20 for wirelesscommunications and position finding, in accordance with an embodiment ofthe invention. By way of example, FIG. 1 shows a typically indoor orurban environment, in which multiple access points 22, 24, 26, . . . ,are deployed, often by different WLAN proprietors independently of oneanother. Signals from the access points are received by mobile stations28, 30, . . . , which are operated by users 32 who are free to movearound within system 20. In the pictured embodiment, stations 28, 30, .. . , are shown as smartphones; but other sorts of mobile devices, suchas laptop and tablets computers, may be used in similar fashion and cansimilarly map angles of departure of access points 22, 24, 26, . . . ,as described hereinbelow. Assuming access points 22, 24, 26, . . . , insystem 20 are compliant with the 802.11n standard, each access point hastwo or three antennas 34, as shown in FIG. 1. The principles of thepresent invention are similarly applicable to 802.11ac access points,which may have an even greater number of antennas. Mobile stations 28,30, . . . , are each assumed to have a single, omnidirectional antenna36, although the techniques described herein for mapping angles ofdeparture can similarly be implemented by multi-antenna stations.

Mobile stations 28, 30, . . . , process signals received from antennas34 in order to estimate the angles of departure of the signals from therespective access points 22, 24, 26, . . . , as well as to extractidentifying information (such as the BSSID) with regard to each accesspoint. The mobile stations are able to perform these functions, asdescribed further hereinbelow, without necessarily associating with theaccess points.

On the other hand, mobile stations 28, 30, . . . , may associate withone or more of access points 22, 24, 26, . . . , for purposes ofInternet communications. Alternatively or additionally, the mobilestations may access the Internet via a cellular network or otherconnection. In any case, mobile stations 28, 30, . . . , communicate theangle-of-departure data and access point identification that theycollect via a network 38 to a mapping server 40. In addition, the mobilestations may communicate their current location coordinates to themapping server, as derived, for example, from GPS signals or from knownlocations of access points or base stations that are provided by server.This information may be collected and reported autonomously andautomatically by a suitable application program (“app”) running in thebackground on the mobile stations.

Server 40 typically comprises a general-purpose computer, comprising aprogrammable processor 42 and a memory 44. The functions of server 40that are described herein are typically implemented in software runningon processor 42, which may be stored on tangible, non-transitorycomputer-readable media, such as optical, magnetic or electronic memorymedia.

Based on the angle-of-departure information, access pointidentification, and location coordinates communicated over network 38 bymobile stations 28, 30, . . . , processor 42 builds up a map of accesspoint locations and orientations in memory 44. As greater numbers ofusers 32 download the application program and convey information toserver 40, the map will grow in both geographic extent and accuracy ofthe access point data, by a process of bootstrapping from an initialbase of seed information. On the basis of this map, server 40 can alsoprovide location and navigation information to users 32 via theapplication program running on their mobile stations, based on theaccess point signals received by the mobile stations at any given time.

Frequency-Based Methods for Estimating Angle of Departure

FIG. 2 is a diagram that schematically illustrates a coordinate frameused in deriving an angle of departure of wireless signals from accesspoint 24 to mobile station 28, in accordance with an embodiment of theinvention. This particular pair of access point and mobile station isselected purely for convenience, and similar principles will apply toany given pair. Although access point 24 is shown as having two antennas34 (labeled Tx1 and Tx2), the same geometrical principles apply toaccess points having three or more antennas arranged in a linear array.

Antennas 34 define an array axis as the line passing through the basesof the antennas. The antennas are separated along the array axis by aknown distance d, which is typically designed to be a half wavelength,for example, λ/2=6.25 cm at the standard WLAN frequency of 2.4 GHz. Theangle of departure θ of the signals from antennas 34 to antenna 36 ofmobile station 28 is taken relative to the normal to the array axis, asshown in FIG. 2. Assuming the distance from access point 24 to mobilestation 28 to be considerably greater than d, there will be a differenceof d*sinθ in the path length from Tx1 to antenna 36 (referred to as Rx)relative to the path length from Tx2.

As an example, assuming the length of the path from Tx2 to Rx is 6.0000m, θ=30°, the slightly longer path from Tx1 to Rx will be 6.03125 m.This path difference translates into a 90° phase difference:Δϕ=dsin(π/6)=L/2*λ/2=λ/4. The propagation delay across 6 m is L/C=6m/(0.3 m/nsec)=20 nsec. Both paths, from Tx1 and Tx2, experience alinear phase shift as a function of frequency, which is assumed to bezero at f₀=2.412 GHz and linearly grows to ϕ=(360°*20 nsec)/50 nsec=144°at f₀=2.432 GHz, wherein T=50 nsec= 1/20 MHz is given by the differenceB=20 MHz between the two frequencies. This linear phase shift of ϕ=144°between the two frequencies on both paths is in addition to the constantphase shift of 90° for the longer path at both frequencies, due to thepath length difference. (Actually, the phase shifts, both linear andconstant, are very slightly larger at 2.432 GHz, since the wavelength isvery slightly longer, but this effect can be neglected since B«f₀)

In addition, in accordance with the 802.11n standard, different cyclicshifts will be applied to the OFDM signals transmitted by Tx2 relativeto Tx1. A cyclic shift of Tcs<0 nanoseconds is equivalent to multiplyingfrequency bin k by

$e^{j\frac{2\pi}{N}nk},$

wherein n=−B*Tcs, and N=64 is the number of frequency bins. It isadvantageous to choose the bins, as explained below, to have anantipodal relation, given the standard cyclic shifts, meaning that thereis a perfect 180° shift between Tx1 and Tx2 in the bin in question.Similarly, in three-antenna constellations, the standard specifiesdissimilar cyclic shifts that are to be applied on Tx2 and Tx3 relativeto Tx1. From the point of view of Rx antenna 36, there is no way ofknowing in advance which antenna 34 is Tx1 and which is Tx2 (or Tx3).There are two possible physical constellations of two antennas (2Tx):(1,2) and (2,1), i.e., the antenna array may be flipped over relative tothe receiver. There are six possible three-antenna (3Tx) constellations:(2,1,3), (1,2,3), (1,3,2) and their flip versions. In general, allpossible constellations are taken into account in computing the angle ofdeparture.

According to the 802.11 standards employing OFDM PHYs, data framestransmitted by access points have a preamble that includes a “ShortTraining Field” (STF) and a “Long Training Field” (LTF), containingpredefined sequences of symbols that are specified by the standards. The802.11a standard defined a frame format that is now referred to as the“legacy” format, which includes a legacy (L) preamble with L-STF andL-LTF fields. The 802.11n standard defines a new format, known as“high-throughput” (HT), with HT-STF and HT-LTF fields. Access pointsoperating in accordance with the 802.11n standard may transmit frames ineither legacy mode, HT-mode (“greenfield”), or mixed mode, in whichframes include both legacy and HT training fields.

The 802.11n standard defines the following cyclic shift values (Tcs) pertransmitting antenna in the legacy and HT preambles (see Tables 20-9 and20-10 in the standard). These Tcs values in turn give rise to differentsubcarrier phase shifts, in accordance with the formula presented above

$\left( e^{j\frac{2\pi}{N}nk} \right),$

for different time-frequency bins (n,k), with N=64 for 20 MHz channels:

TABLE 1 Legacy HT Tcs n k = 0 k = 4 k = 8 k = 16 Tx1 Tx1 0 0 0 0 0 0 Tx2— −100 2 0 π/4 π/2  π Tx3 Tx3 −200 4 0 π/2  π 2π — Tx2 −400 8 0 π 2π 4π

Antipodality between the selected bins, i.e. 180° phase shift followingCDD, provides the greatest Euclidean distance between the signals, thusenhancing noise immunity.

Embodiments of the present invention use two different methods toachieve antipodality:

(1) In the time domain method, a bin k0 in a legacy OFDM symbol and abin k0 in an HT symbol are selected at a certain fixed frequency.

(2) In the frequency domain method, two bins are chosen at differentfrequencies in a single OFDM symbol, for example a legacy OFDM symbol.

Antipodality provides the best Euclidean distance for noise immunity.

Projecting the path differences between the antennas back to thetransmitter antenna feeds, the phases of the two signals are (0, θ), orequivalently (e^(j0), e^(jθ)) in complex form, for the two-antenna case,and (−θ, 0, θ), equivalent to (e^(−jθ), e^(j0), e^(jθ)), in thethree-antenna case, wherein 0 is the angle of departure as shown in FIG.2. Projecting post-IFFT encoder cyclic shifts onto the transmitter“frequency domain,” for judiciously chosen shifts and bins, gives aphase of either 0 or 180°. The two or more rays emitted from thetransmitter are superimposed at the single antenna of the receiver.

The combined effect of path propagation, cyclic shift and superpositionis a signal of the form

1±e^(jθ) for two antennas. For three antennas, there are two physicallydifferent cases: an anti-symmetrical case (“case I”) and a symmetricalcase (“case II”), yielding a combined signal of e^(−jθ)+1±e^(jθ) ore^(−jθ)±1+e^(jθ), respectively.

Returning now to the specific two-antenna example shown in FIG. 2 andthe path phase differences calculated above, selecting natural exponentsof jπ in the time domain (as indicated by the “π” entries in Table 1),the phase shifts between the signals received by antenna 36 fromantennas 34 Tx1 and Tx2 at f₀=2.412 MHz in the L-LTF and HT-LTF fieldsof a mixed-mode frame will be as follows:

Tx L- LTF HT-LTF 1  0° 0° 2 −90° −90° + 180°In the above example, the difference of −90° in the antenna pathsapplies only to Tx2, while 180° is due to the cyclic shift, which isapplied only to HT-LTF on Tx2.

The phases of the corresponding received signals at f₁=2.432 MHz willbe:

Tx L-LTF HT-LTF 1  0° + 144° 0° + 144° 2 −90° + 144° −90° + 144° + 180°Here the additional 144° is due to the phase shift across the commonpath from transmitter to receiver, applied to all time slots.

Upon receiving the signals from the transmitted L-LTF bins in thefrequency domain case, or both HT-LTF and L-LTF bins in the time domaincase, from access point 24, mobile station 28 computes the phasedifference between the Tx1 path and the Tx2 path. This difference isindicative, in turn, of the angle of departure of the signals fromaccess point 24, as illustrated in FIG. 2. Computational methods thatcan be used to derive the angle of departure in this manner aredescribed hereinbelow.

FIG. 3 is a block diagram that schematically illustrates components ofmobile station 28 that are used in deriving coordinate information withrespect to wireless access points, in accordance with an embodiment ofthe invention. The description that follows assumes that mobile station28 has a single antenna 36, which receives the signal streamstransmitted by two or three antennas 34 of an access point, but theprinciples of this embodiment may similarly be applied, mutatismutandis, by a multi-antenna receiver in order to achieve path diversitywhile measuring the same angle of departure. As explained above, theanalysis performed by mobile station 28 relies on the fact that thesignals transmitted from the different antennas encode identical data inthe frame preambles, with a predefined cyclic delay between thetransmitted signals as defined by the 802.11n standard.

A front end (FE) circuit 50 in mobile station 28 amplifies, filters, anddigitizes the signals received by antenna 36, as is known in the art,and passes the resulting digital samples to digital processing circuitry52. A fast Fourier transform (FFT) circuit 54 in circuitry 52 dividesthe incoming signal into time-frequency bins (n,k), wherein eachfrequency corresponds to a different OFDM subcarrier, and the phase ofthe signal component in each bin is determined by the data value that itencodes. A media access control (MAC) processing circuit 56 extractsdata from the frame header, including the BSSID that identifies theaccess point that transmitted the frame. Circuits 50, 54 and 56, as wellas other components of digital processing circuitry 52, are conventionalelements of 802.11 receivers, such as those installed in WiFi-capablesmartphones that are known in the art for purposes of data reception andtransmission. Other elements of the receiver that are not essential foran understanding of the present invention are omitted for the sake ofbrevity. The signal complex frequency bins generated by FFT circuit 54are input to an angle estimation block, which includes a bin selector 58and a transformation module 60 and an angle differentiator 61. Theseelements convert the complex values into an estimated angle ofdeparture. They are typically implemented in software running on aprogrammable processor in mobile station 28. This software may be a partof an application program running on the mobile station, as describedabove, along with other functions performed by a processor alreadypresent in the mobile station. This program may be stored in tangible,non-transitory computer-readable media, such as optical, magnetic, orelectronic memory media. Alternatively or additionally, at least a partof the angle estimation functions described herein may be performed byin dedicated or programmable hardware logic. Bin selector 58 selects apair of time-frequency bins, (n₁,k₁) and (n₂,k₂), to be extracted fromthe preambles of the received signals, and extracts the correspondingphase values. For each bin, the bin selector computes the complexvectors, y_(i), between the respective signals received from two ofantennas 34. The two selected complex bins define a complex vector

${\overset{\rightarrow}{y} = \begin{bmatrix}y_{1} \\y_{2}\end{bmatrix}},$

which is input to complex transformation module 60. Although any pair ofbins can be chosen for this purpose, it is advantageous to choose apair, based on the known cyclic shifts, in which the relative phasedelay can be linearly related to the angle of departure of the signalstransmitted from the access point across a broad span of angles. Thechoice of the difference between the frequencies of the bins, Δk=k₁−k₂,involves a tradeoff between thermal noise, which can be significant forsmall Δk, and channel deviations and sensitivity to time of flight,which grow as Δk grows. The numerical examples provided here assumecyclic shifts are chosen so as to achieve antipodality. The frequencyresponse of the wireless channel between the access point and thereceiver is not linear in phase, due, for example, to the impact ofreflections. In general, the larger the value of Δk, the larger theadded error to the estimated phase delay due to variation in the channelresponse. In the case of 802.11n signals with 20 MHz bandwidth, forexample, a reasonable tradeoff is achieved with Δk=8, which reflects afrequency difference of 8/64*20 MHz=2.5 MHz. The following frequencycombinations (k₁,k₂) satisfy this criterion: (0,8), (16,8), (16,24),(32,40), (48,40), (48,56) and (0,56). Since the HT-LTF does not populatefrequencies 0 and 32, however, the combinations (0,8) and (32,40) arenot applicable. Other bin pair patterns of the form (k₀,k₀₊₈) can beused, as well, so long as both preamble bins carry energy.

Transformation module 60 applies a linear transformation T in order toconvert the complex input vector into an output vector: {right arrowover (x)}=T{right arrow over (y)}. The purpose of this transformation isto generate the pair of values

$\overset{\rightarrow}{x} = \begin{bmatrix}x_{1} \\x_{2}\end{bmatrix}$

such that the phases of x₁ and x₂ can be easily extracted and subtractedby differentiator 61 to give the estimated angle of departure:{circumflex over (θ)}=arg(x₂)−arg(x₁).

For example, in the case of two antenna elements the followingtransformation provides linear estimation of the output vector:

$\overset{\rightarrow}{x} = {\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}\overset{\rightarrow}{y}}$

Mobile station 28 transmits the estimated angle of departure, along withthe BSSID extracted by MAC processing circuit 56, via network 38 toserver 40. In addition, mobile station 28 transmits informationregarding the current location of the mobile station to server 40, asprovided by a location resolution module 62. Module 62 may comprise, forexample, a GPS receiver, which outputs location coordinates based onsatellite signals. Additionally or alternatively, module 62 may beimplemented in software running on a processor in processing unit 52 andmay compute current location coordinates of mobile station 28 based onsignals received from other, known access points that have already beenmapped by server 40, or other sources. In any case, server 40 receivesthe estimated angle of departure data together with the current,estimated location of the mobile station. The server is thus able tofind the actual location of access point 24 based on multiple anglemeasurements reported by mobile station 28 (or by multiple mobilestations) from different locations.

Examples of Angle f Departure Estimation

Two Antennas, Mixed Format (MF, HT with Legacy Compliance) Mode

A plausible “time domain” choice of a time-frequency pair of bins in thecase of two transmitting antennas is (n,k)=(4,k₀) and (8,k₀), wherein k₀is any element in the set {8,24,40,56}. This choice providesantipodality, i.e., the phase contribution of the cyclic shift is either0 or 180°.

The time bin n=4 is in the legacy preamble (L-LTF), while bin n=8 (400nsec*20 MHZ) is in the HT preamble (HT-LTF). (The bin n=4 corresponds to200 ns*20 MHZ, wherein 200 ns is the CDD mandated by the standard, and20 MHz is a typical 802.11 OFDM channel bandwidth. Channel bandwidths of40 MHz, 80 MHz and 160 MHz are possible, as well.) This selection ofbins generates antipodal phases between the two readings, i.e., summing(or equivalently, subtracting) the signals from the two antennas in themeasured bins gives the complex vector

$\overset{\rightarrow}{y} = {\begin{bmatrix}{1 + e^{j\;\phi}} \\{1 - e^{j\;\phi}}\end{bmatrix}.}$

Transformation module 60 transforms this vector linearly into

$\overset{\rightarrow}{x} = {{\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}\overset{\rightarrow}{y}} = {{2\begin{bmatrix}1 \\e^{{- j}\;\phi}\end{bmatrix}}.}}$

Hence the estimated angle of departure is {circumflex over(θ)}=arg(x₂)−arg(x₁)=ϕ.

Three Antennas, MF Mode

Case I The same time-frequency bins are selected as in the previousexample, assuming that antennas 34 are arrayed along the axis of thetransmitting access point in the order (1,3,2) or (2,1,3) or in the flipversions of these orders. In all of these cases, extraction of thephases from the pairs of bins (n,k)=(4,k₀) and (8,k₀) will yield thefollowing complex vector:

$\overset{\rightarrow}{y} = {\begin{bmatrix}{e^{{- j}\;\phi} + 1 + e^{j\;\phi}} \\{e^{{- j}\;\phi} + 1 - e^{j\;\phi}}\end{bmatrix}.}$

The reason for this result is that antennas Tx1 and Tx3 have the samecyclic shift (n=4), while Tx2 has a cyclic shift of n=8, which for theabove set of values of k₀ results in a complete 180° rotation.

Applying the same transformation in module 60 as in the precedingexample results in the angular vector:

$\overset{\rightarrow}{x} = {{\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}\overset{\rightarrow}{y}} = {{2\begin{bmatrix}{1 + e^{j\;\phi}} \\e^{j\;\phi}\end{bmatrix}} = {{e^{{- j}\frac{\phi}{2}}\begin{bmatrix}{e^{{- j}\frac{\phi}{2}} + e^{j\frac{\phi}{2}}} \\e^{j\frac{3\phi}{2}}\end{bmatrix}} = {{e^{{- j}\frac{\phi}{2}}\begin{bmatrix}{\cos\frac{\phi}{2}} \\e^{j\frac{3\phi}{2}}\end{bmatrix}}.}}}}$

Hence,

$\theta = {{{\arg\left( x_{2} \right)} - {\arg\left( x_{1} \right)}} = {\frac{3\phi}{2}.}}$

Case II

The array constellation of (1,2,3) and its flip case result in thefollowing phase vector

$\overset{\rightarrow}{y} = {\begin{bmatrix}{e^{{- j}\;\phi} + 1 + e^{j\;\phi}} \\{e^{{- j}\;\phi} - 1 + e^{j\;\phi}}\end{bmatrix}.}$

In this case, module 60 applies the transformation

${\overset{\rightarrow}{x} = {\begin{bmatrix}1 & {- j} \\1 & j\end{bmatrix}\overset{\rightarrow}{y}}},$

which generates a non-linear S-curve of angle as a function of phase,spanning

$\frac{3\pi}{2}.$

The curve is symmetrical around the boresight direction (i.e., thedirection perpendicular to the array axis shown in FIG. 2). In otherwords, for a given phase delay, the angle of departure has a unique,corresponding absolute value, with a sign that can be either positive ornegative.

Similar results can be obtained by a frequency domain analysis: Whenusing different frequency bins of the HT-LTF, the signal from eachantenna is cyclically shifted by a different delay, as shown in Table 1above. The case of Δk=8 is identical in phase shifts to thethree-antenna time-domain case analyzed above, with phase shifts of 0and π:

$e^{j\frac{2\pi}{N}nk}$   n   Tcs   k = 16   k = 8 e^(−jθ) 0   0 1 1e^(j0) 8 −400 1 1 e^(+jθ) 4 −200 1 −1 

The bins (n,k)=(8,0) and (n,k)=(8,8) result in the same phasecombination as the above time domain case I:

${\overset{\rightarrow}{y} = \begin{bmatrix}{e^{{- j}\;\phi} + 1 + e^{j\;\phi}} \\{e^{{- j}\;\phi} + 1 - e^{j\;\phi}}\end{bmatrix}},$

with the same angle estimator as above. The resultant estimator isperfectly linear with output span of 3π, and a full span set at a slantof π.

Three Antennas, Legacy Mode

In legacy mode, access point 24 periodically transmits beacons,typically roughly every 100 msec. Therefore, mobile station 28 canadvantageously receive and process these beacons to find the angle ofdeparture of access point 24, since the beacons are transmittedirrespectively of any association by a mobile station. Furthermore,because beacons are always sent with a bandwidth of 20 MHz, the receivercan be agnostic to the actual bandwidth that is used for data (which maybe 20, 40, 80 or 160 MHz, for example).

In the case of the L-LTF, the cyclic shift relative to Tx1 is 100 ns forTx2 and 200 ns for Tx3, respectively:

n Tcs k = 0 k = 16 e^(−jθ) 0 0 1 1 e^(j0) 2 −100 1 −1 e^(+jθ) 4 −200 1 1Hence for Δk=16 (double the frequency offset Δk=8 used above for HT),the phase pattern is identical to Case I above.

Antenna Constellation

The transformations described above, when applied by transformationmodule 60, give estimates of the angle of departure but do notnecessarily resolve the actual configuration of the transmit antennaconstellation. Digital processing unit 52 may therefore apply anadditional, parallel estimator in order to differentiate between one,two and three transmitting antennas, and in the case of three antennas,to differentiate between Case I and Case II defined above. In oneembodiment of the present invention, transformation module 60 extractsthe antenna constellation by applying a different linear transformationin the time domain: No transformation in the three-antenna Case IIresults in an output difference of either 0 or π at any direction ofdeparture. In the case of two antennas, the following transformationresults in an output difference of either 0 or π at any direction ofdeparture:

$\overset{\rightarrow}{x} = {\begin{bmatrix}1 & {- j} \\1 & j\end{bmatrix}{\overset{\rightarrow}{y}.}}$

A second method to ascertain the number of antenna elements and theirlinear formation is based on using a plurality of time-frequency pairs(n, k), all of which provide an unbiased time of departure estimation.For example, in the frequency domain method described above with bins(n,k₀) and (n,k₀₊₈), values of k₀=0, 1, 2, . . . , 55 can be used forbins bearing energy n=4 and N=64.

It is not generally possible to determine whether in the two-antennacase, the antenna order is (1,2) or (2,1). The ambiguity of order can bereadily resolved, however, by making multiple measurements of angle ofdeparture, using the same receiver or multiple different receivers atdifferent locations.

Three-antenna transmitters can readily be distinguished from two-antennatransmitters using the techniques described above: Since differentcyclic delays are used in two- and in three-antenna devices, receptionand processing of a single MF packet is sufficient to apply a crosscheckbetween the above time-domain and frequency-domain estimations. Only thecorrect antenna estimator (two-antenna or three-antenna) will give thesame results for both the time-domain and frequency-domain estimations.

Correlation-Based Methods for Estimating Angle of Departure

FIG. 4 is a block diagram that schematically illustrates components ofmobile station 28 that are used in deriving coordinate information withrespect to wireless access points, in accordance with an alternativeembodiment of the invention. As in the preceding embodiments, thedescription that follows assumes that mobile station 28 has a singleantenna 36, which receives the signal streams transmitted by two, threeor more antennas 34 of an access point. The analysis performed by mobilestation 28 relies on the fact that the signals transmitted from thedifferent antennas encode identical data in the frame preambles, with apredefined cyclic delay between the transmitted signals as defined bythe applicable standard, such as 802.11n, 802.11ac, or other standardsthat have been or may be promulgated in the future. Elements of themobile station shown in FIG. 4 that have the same structure andfunctionality as corresponding elements in the embodiment of FIG. 3 arelabeled with the same reference numbers and are omitted from thedescription that follows for the sake of brevity. The present embodimentmakes use of synchronization symbols that are present in the preamble ofeach frame transmitted by a transmitter, such as access point 24 (FIG.2), in accordance with the 802.11n standard. These synchronizationsymbols have good correlation qualities, meaning that a correlationfunction computed over the frame has sharp, well-defined peaks. Amongthe synchronization symbols, the legacy Short Training Field (STF) issent first. The exact timing of this field can be detected using a shortcorrelator and is effective in estimating coarse frequency offsets. Thesubsequent legacy Long Training Field (LTF) requires a correlator thatis five times longer and is also used for symbol alignment and channelestimation.

The signals originating from different transmitter antennas 34 arealmost completely overlapping in time: A typical packet duration isabout 200 μs, while the cyclic shift amounts to only about 200 ns, onethousandth as long. The decorrelation time of the LTF, however, is evenshorter and is physically limited by the channel bandwidth, which is atleast 20 MHZ, corresponding to 50 ns in time. The legacy LTF comprisestwo and a half repetitions of a 64-element complex vector, totaling 160time samples at 20 MHz. A cross-correlation between the signals receivedby antenna 36 and an LTF reference signal will thus exhibit two strongpeaks, 64 samples apart, and weaker peaks further away. The legacy STFcomprises ten repetitions of a sixteen-element complex vector, totaling160 time samples at 20 MHz and giving rise to many cross-correlationpeaks.

In the case of 802.11n, as explained above, the same data aretransmitted by all antennas 34, with a unique cyclic shift per antenna.The legacy cyclic shift for two antennas is 200 ns, which is equivalentto a spacing of four samples at 20 MHz. (Other cyclic shift values arelisted above in Table I.) A correlator can differentiate between thesignals transmitted by different antennas carrying identical data (forexample, synchronization symbols) as long as the cyclic shift betweenthe signals is larger than the decorrelation time, which is 50 ns in thepresent example. Thus, a correlation function computed at the receiver(mobile station 28) will have a peak for each transmitting antenna 34,separated by the exact relative cyclic shift between the antennas(assuming channel multipath effects are small, as would be expectedindoors, for example).

Digital processing circuitry 70 in the embodiment of FIG. 4 makes use ofthese correlation properties in order to extract the phase differencebetween the signals transmitted by antennas 34, and hence the angle ofdeparture θ (as shown in FIG. 2). A correlator 72 applies a correlationfunction to the digitized signals that are output by front end 50. Forexample, correlator 72 may compute a time-shifted auto-correlation ofthe digitized signal or may compute a cross-correlation between thedigitized signal and a reference. A peak analyzer 74 identifies thepeaks in the correlation and is thus able to ascertain the cyclic shiftbetween the signals transmitted by the different antennas. For example,a correlation function computed over the legacy LTF in the 802.11npreamble, as explained above, will have a pair of peaks 64 samples apartdue to the repetition of the 64-element LTF symbol vector, along withadditional peaks displaced by four samples as a result of the cyclicshift of −200 ns between the antennas.

Correlator 72 multiplies the complex input received from front end 50 bythe conjugate of the known signal and integrates over a few tens ofsamples. A peak analyzer 74 measures the epoch of the N strongest peaksin a given time window in the correlator output and attempts to locatethe expected pattern of epochs four samples apart (due to the knowncyclic shift of −200 ns) and epochs 64 samples apart (due to theinherent repetition of the transmitted legacy LTF symbol, with 2.5replicas of 64 samples, totaling two symbols and 160 samples). Thecombined pattern for two antennas will thus give peaks at locations {0,4, 64, 68}. Peak analyzer 74 can also handle situations in which somepeaks are missing (for example, when the transmitter has only oneantenna) and in which some peaks are phantoms (not originating from thetransmitter of interest or sidelobes of the actual signal from thetransmitter).

In one embodiment of the present invention, peak analyzer 74 applies atwo-stage procedure:

Peak analyzer 74 first applies Symbol Alignment (SA) to decide whichinput sample is the first sample in the first symbol of the packet. SAlooks for the legacy LTF repetition of 64 symbols, as explained above.Peak analyzer 74 may optionally be configured to tolerate up to onesample error in time, e.g., a time difference of 63 or 65 samplesinstead of the nominal 64 may be accepted, since the received signal istime-sampled at the exact transmitter clock rate but is not aligned withit (and may thus be skewed by up to half a clock cycle).

To carry out the SA stage, peak analyzer 74 may apply the followingalgorithm: Pick the four strongest peaks in a predefined time window,n0, n1, n2 and n3 in ascending order in time (not signal strength).Define a “good pair” as a pair of epochs that are 63, 64 or 65 samplesapart.

A: If (n3,n1) are good then pick n3.

B: Else if (n2,n0) are good then pick n2.

C: Else, if (n2,n1) are good then pick n2.

D: Else, if (n3,n0) are good then pick n3.

E: Else, if (n1,n0) are good then pick n1.

A is an arbitrary decision; C conjectures the third peak was too weak; Dconjectures there is a phantom signal in between; E conjectures thetransmitter has only one antenna.

Once symbol alignment is established, so that the first sample of thefirst symbol is identified, processing circuitry 70 can find the carrierphase difference between the two early peaks at sample locations {0,4}if both are present, and/or the carrier phase difference between the twolater peaks at sample locations {64,68} if both are present. This phasedifference is used to estimate the Direction of Departure, as explainedbelow. The two estimates can also be combined for improved accuracy, forexample by averaging.

Direction of Departure (DD) detection: Based on successful SA, asexplained above, if either pair of peaks {0,4} or {64,68} survives(i.e., either both of the peaks at locations 0 and 4 or both of thepeaks at locations 64 and 68, or better yet, all four peaks, are strongenough to be considered), an angle of departure can be extracted. Inthis case, the DD algorithm, extracts the angle of departure fromwhichever pair or pairs of peaks are available. When both pairs areavailable, the corresponding estimates can be reduced to a single,better estimate, for example by averaging or filtering.

Phase extraction circuit 76 uses the relative peak locations inestimating the carrier phase of each of the copies of the transmittedsignal, such as the copies transmitted respectively by antennas Tx1 andTx2 in FIG. 2. The phases between the two signals are offset by theshift due to the path difference dsinθ. Assuming the signals received atantenna 36 from Tx1 and Tx2 are roughly equal in strength (to avoidconfusion with correlation sidelobes), circuit 76 can use thecorrelation between the signals and appropriate-shifted versions of theLTF to perform separate carrier phase estimations for the signals fromboth of the antennas. The carrier phase differences are indicative ofthe small path difference dsinθ and can thus be used by circuit 76 inextracting the angle of departure θ. Alternatively, any other suitablesymbol that is known to be in the transmitted signals can be used inthis manner (in original and cyclic-shifted versions) to estimate thephase difference between the signals.

Processing circuitry 70 does not typically have advance informationregarding the constellation of transmit antennas 34. For example, accesspoint 24 may comprise either two antennas, as shown in FIG. 2, or threeor more antennas. Peak analyzer 74 can resolve the number of antennas inthe array of the access point by finding the numbers and relativelocations of the peaks in the correlation function computed bycorrelator 72.

Furthermore, in a linear array of antennas, the cyclic shifts can beapplied to the antennas in any order along the line. In the two-antennacase, swapping the order of the antennas will result in anindistinguishable mirrored angle estimation. With more than twoantennas, when mobile station 28 is located in the far field, processingcircuitry 70 is able deduce the actual order of the antennas in terms ofthe cyclic shifts and the phase differences. Thus, processing circuitry70 will be able to distinguish, based on the relative cyclic shifts,between an array in which the order of the antennas is 1-2-3 and one inwhich the order is 1-3-2, but still will not be able to distinguishbetween 1-2-3 and 3-2-1. The missing information can be filled in bymaking measurements of signals from the same access point 24 atdifferent receiver locations.

System Applications

FIG. 5 is a schematic, pictorial illustration of components of thesystem of FIG. 1, illustrating a method for finding the position of amobile communication device 80, in accordance with an embodiment of theinvention. This method assumes that the respective location coordinatesand BSSIDs of access points 22, 24 and 26 have already been mapped byserver 40, on the basis of measurements of angle of departure that weremade previously by other mobile stations and/or other input data.

Device 80 receives multi-antenna signals from each of access points 22,24 and 26 and extracts the respective angle of departure for each accesspoint, labeled θ₁, θ₂, and θ₃ in the figure, using the techniquesdescribed above, along with the respective BSSIDs. Device 80 reportsthese findings via network 38 to server 40, which returns correspondinglocation coordinates. The server may return the location coordinates ofthe access points, in which case device 80 can triangulate its ownposition based on these coordinates and the measured angles ofdeparture. Alternatively or additionally, device 80 conveys the valuesof the angles of departure that it has estimated to server 40, whichthen returns the location coordinates of device 80.

In order to provide these sorts of location data to device 80, server 40builds up and maintains a map of access point locations and orientationsin memory 44 (FIG. 1). Typically, the map is built up on the basis ofmeasurements of angle of departure, BSSID, and device location that arereported by mobile stations in various areas. Server 40 may use theinformation reported by device 80, as illustrated in FIG. 4, not only toprovide location information to device 80, but also to extend and refinethe map maintained by the server. In this manner, for example, theserver can continually add new access points to the map and can refinethe accuracy of the access point locations and orientations in the map.

Server 40 can build this access point map without requiring anycooperation by operators of the access points. Similarly once users ofmobile devices have installed the mapping application, their mobiledevices can measure and report access point data to the serverautonomously, without active user involvement. The more users subscribeto the mapping application, the more extensive and more accurate will bethe resulting maps and access point locations that they provide.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. A method for signal processing, comprising: receiving at a givenlocation at least first and second signals transmitted respectively fromat least first and second antennas of a wireless transmitter, the atleast first and second signals encoding identical data using amulti-carrier encoding scheme with a predefined cyclic delay between thetransmitted signals; processing the received first and second signals,using the cyclic delay, in order to derive a measure of a phase delaybetween the first and second signals; and based on the measure of thephase delay, estimating an angle of departure of the first and secondsignals from the wireless transmitter to the given location.
 2. Themethod according to claim 1, wherein receiving the at least first andsecond signals comprises receiving at least the first and second signalsvia a single receiving antenna.
 3. The method according to claim 2,wherein the single receiving antenna is an omnidirectional antennainstalled in a mobile telephone.
 4. The method according to claim 1,wherein the multi-carrier encoding scheme comprises an orthogonalfrequency-domain multiplexing (OFDM) scheme.
 5. The method according toclaim 4, wherein the transmitter is a wireless access point operating inaccordance with an 802.11 standard. 6-15. (canceled)
 16. The methodaccording to claim 1, wherein the first and second signals aretransmitted in accordance with a wireless communication standard thatspecifies a frame structure including a predefined preamble, and whereinprocessing the received first and second signals comprises selecting atleast a part of the preamble of a given frame for processing.
 17. Themethod according to claim 1, wherein the transmitter is a wirelessaccess point, and wherein receiving and processing the at least firstand second signals comprise receiving and processing at least the firstand second signals in a mobile station without establishing anassociation between the mobile station and the access point. 18.(canceled)
 19. The method according to claim 1, wherein the transmitteris a wireless access point, and the method comprises receiving locationinformation with respect to the wireless access point, and computingcoordinates of the given location based on the received locationinformation and the estimated angle of departure.
 20. (canceled)
 21. Themethod according to claim 1, wherein the transmitter is a wirelessaccess point, and the method comprises extracting an identifier of thewireless access point from at least one of the received first and secondsignals.
 22. (canceled)
 23. A method for mapping, comprising: receivingreports, from a set of wireless communication devices at respectivelocations, of respective estimated angles of departure of signalsreceived by the wireless communication devices from wireless accesspoints; and constructing a map of the wireless access points based onthe estimated angles of departure.
 24. The method according to claim 23,wherein the wireless communication devices comprise mobile stationshaving a single antenna, while the wireless access points each havemultiple antennas, and wherein the angles of departure are estimatedbased on a predefined cyclic delay between the signals transmitted bythe multiple antennas.
 25. The method according to claim 23, whereinreceiving the reports comprises receiving from the wirelesscommunication devices respective identifiers of the wireless accesspoints and location coordinates of the wireless communication deviceswhere the signals were received.
 26. The method according to claim 23,and comprising providing location information from the map to one ormore of the wireless communication devices.
 27. The method according toclaim 26, wherein providing the location information comprisesidentifying position coordinates of a wireless communication devicebased on an estimated angle of departure of signals received by thewireless communication devices from a given access point and a locationof the given access point in the map.
 28. A wireless device, comprising:a receive antenna, configured to receive at a given location at leastfirst and second signals transmitted respectively from at least firstand second antennas of a wireless transmitter, the at least first andsecond signals encoding identical data using a multi-carrier encodingscheme with a predefined cyclic delay between the transmitted signals;and processing circuitry, which is configured to process the receivedfirst and second signals, using the cyclic delay, in order to derive ameasure of a phase delay between the first and second signals, and toestimate, based on the measure of the phase delay, an angle of departureof the first and second signals from the wireless transmitter to thegiven location.
 29. The device according to claim 28, wherein thereceive antenna comprises a single antenna, which receives both thefirst and second signals.
 30. The device according to claim 29, whereinthe single antenna is an omnidirectional antenna, and the device is amobile telephone.
 31. The device according to claim 28, wherein themulti-carrier encoding scheme comprises an orthogonal frequency-domainmultiplexing (OFDM) scheme.
 32. The method according to claim 31,wherein the transmitter is a wireless access point operating inaccordance with an 802.11 standard. 33-42. (canceled)
 43. The deviceaccording to claim 28, wherein the first and second signals aretransmitted in accordance with a wireless communication standard thatspecifies a frame structure including a predefined preamble, and whereinthe processing circuitry is configured to select at least a part of thepreamble of a given frame for processing.
 44. The device according toclaim 28, wherein the transmitter is a wireless access point, andwherein the processing circuitry is configured to receive and process atleast the first and second signals without establishing an associationwith the access point.
 45. (canceled)
 46. The device according to claim28, wherein the transmitter is a wireless access point, and wherein theprocessing circuitry is configured to receive location information withrespect to the wireless access point, and to compute coordinates of thegiven location based on the received location information and theestimated angle of departure.
 47. (canceled)
 48. The device according toThe device according to claim 28, wherein the transmitter is a wirelessaccess point, and wherein the processing circuitry is configured toextract an identifier of the wireless access point from at least one ofthe received first and second signals.
 49. (canceled)
 50. Apparatus formapping, comprising: a memory; and a processor, which is configured toreceive reports, from a set of wireless communication devices atrespective locations, of respective estimated angles of departure ofsignals received by the wireless communication devices from wirelessaccess points, and to construct, in the memory, a map of the wirelessaccess points based on the estimated angles of departure.
 51. Theapparatus according to claim 50, wherein the wireless communicationdevices comprise mobile stations having a single antenna, while thewireless access points each have multiple antennas, and wherein theangles of departure are estimated based on a predefined cyclic delaybetween the signals transmitted by the multiple antennas.
 52. Theapparatus according to claim 50, wherein the reports comprise respectiveidentifiers of the wireless access points and location coordinates ofthe wireless communication devices where the signals were received. 53.The apparatus according to claim 50, wherein the processor is configuredto provide location information from the map to one or more of thewireless communication devices.
 54. The apparatus according to claim 53,wherein the location information identifies position coordinates of awireless communication device based on an estimated angle of departureof signals received by the wireless communication devices from a givenaccess point and a location of the given access point in the map.